Definition
Perpendicularity is a geometric tolerance that controls how close a surface or an axis stands to exactly ninety degrees relative to a reference plane (a datum).
What it means for the calculation
A tight perpendicularity requirement drives the fixturing strategy: it often needs a single set-up to avoid clocking error, or a secondary finishing operation, and both raise cost.
Where it appears on the drawing
In a feature control frame: the ⟂ symbol, a tolerance value and a datum letter, for example [⟂ | 0.02 | A].
Common mistakes
Confusing perpendicularity with a plain ninety-degree angle dimension: perpendicularity is measured against a datum, not just judged by eye. Also picking the wrong reference surface, which changes what the same drawing actually measures.
A tight perpendicularity requirement drives the fixturing strategy: it often needs a single set-up to avoid clocking error, or a secondary finishing operation, and both raise cost.
See how Blake reviews a partFrequently asked questions
Is perpendicularity the same as a ninety-degree angle dimension?
No, an angle dimension checks a nominal angle with a tolerance; perpendicularity checks deviation from an exact datum-referenced plane.
Why does the datum matter?
Because perpendicularity is always relative to a chosen reference surface; change the datum and the same feature can pass or fail.
Can perpendicularity be measured without a CMM?
Sometimes with a height gauge or dial indicator on a surface plate, but tight tolerances usually need a CMM.